The concept of producing a strong, abrasion resistant glass body by developing a thin, crystalline, compressive stress layer in situ thereon by subjecting the glass body to a particularly-defined heat treatment was first described in U.S. Pat. No. 2,998,675 (Olcott et al.). That patent disclosed glass compositions comprising, in parts by weight, 65-72 SiO.sub.2, at least 4 Li.sub.2 O, 22.5-30 Al.sub.2 O.sub.3, the weight ratio Li.sub.2 O:Al.sub.2 O.sub.3 being no greater than 0.3:1, and at least one crystallization catalyst selected from the group 0.1-3.5 TiO.sub.2, 0.1-5 B.sub.2 O.sub.3, 0.4-2 Na.sub.2 O, and 0.5-10 PbO. The patent prescribed exposing the glass body for times ranging from one hour at a temperature equivalent to a glass viscosity of 10.sup.7 poises to 40 hours at a temperature equivalent to a glass viscosity of 10.sup.10 poises. Those temperatures were indicated as ranging from a high of about 870.degree. C. to a low of about 750.degree. C. The crystals generated in the surface layer were stated to be of .beta.-eucryptite which exhibited a lower linear coefficient of thermal expansion than that of the interior glass. Their presence, in turn, reduced the overall coefficient of thermal expansion of the surface layer relative to that of the interior glass, resulting in a uniform compressive stress in and parallel to the surface of the glass article after it is cooled. The establishment of that surface layer imparted greatly enhanced mechanical strength to the glass article.
A second early description of forming such articles can be found in U.S. Pat. No. 3,253,975 (Olcott et al.). Method steps, temperatures, and times similar to those disclosed in U.S. Pat. No. 2,998,675 were employed, but the glass compositions comprised, in parts by weight, 52-65 SiO.sub.2, at least 4 Li.sub.2 O, and not over 40 Al.sub.2 O.sub.3, with the weight ratio Li.sub.2 O:Al.sub.2 O.sub.3 being no greater than 0.3:1. Other compatible oxides were permitted in limited amounts, but none was required. .beta.-eucryptite crystals were again generated in the surface to provide a compression layer.
Another early disclosure of producing such articles is provided in U.S. Pat. No. 3,490,984 (Petticrew et al.) wherein the base glass composition consisted essentially, in weight percent, of 2-4% Li.sub.2 O, 2-10% MgO, 10-20% Al.sub.2 O.sub.3, and 60-75% SiO.sub.2, with at least one member of the group of 0-3% ZrO.sub.2, 0-1.5% TiO.sub.2, and 0-2% SnO.sub.2. Upon heat treatment at temperatures in the vicinity of the softening point of the glass (a glass viscosity of 10.sup.7.65 poises), a thin surface compression layer was generated containing .beta.-eucryptite-like and/or .beta.-spodumene-like crystals.
British Patent Specifications Nos. 1,108,473 and 1,108,476 also describe glass bodies having surface compression layers containing crystals. Both patents disclose glass compositions which consisted, in weight percent, of 24-56% ZnO, 3-25% Al.sub.2 O.sub.3, and 32-54% SiO.sub.2. The preferred compositions contained a nucleating agent selected from the group of 0.5-6% P.sub.2 O.sub.5, 0.5-4% MoO.sub.3, and 1.5-7.5% ZrO.sub.2. Upon heat treatment willemite crystals provide an opaque or opalescent surface layer.
British Patent Specification No. 1,322,796 discloses a modification in the method described in the above two British patents for producing glass bodies having crystal-containing, surface compression layers. The glass compositions were similar to the earlier bodies and the modification in method comprises abrading the surface of the glass body prior to exposing it to a heat treatment. The prior surface abrasion was stated to both improve the strength of the final product and the transparency thereof.
U.S. Pat. No. 4,814,297 (Beall et al.) discloses yet another glass body having a surface compression layer, this layer containing microscopic-sized crystals of .beta.-eucryptite and/or .beta.-quartz solid solution. The thrust of that disclosure was to produce transparent, essentially haze-free bodies through a particularly designed heat treatment of glasses consisting essentially, in weight percent, of 5-7% Li.sub.2 O, 0-2% Na.sub.2 O, 0-10% ZnO, 22-28% Al.sub.2 O.sub.3, and 55-67% SiO.sub.2, the molar ratio R.sub.2 O+RO:Al.sub.2 O.sub.3 &gt;1 and the molar ratio SiO.sub.2 :Al.sub.2 O.sub.3 .apprxeq.4. The preferred glasses contain at least 1% ZnO and, most preferably, contain 4-8% ZnO. The heat treatment contemplates first heating the glass to 650.degree.-850.degree. C. for a sufficient length of time to develop nuclei in a thin surface layer on the glass, and thereafter further heating the surface nucleated glass in the range of 650.degree.-850.degree. C. for a period of time up to two hours to grow the desired crystals on the nuclei. Beall et al. referred to the earlier Olcott et al. patents discussed above, noting that their surface crystallized glass bodies were essentially haze-free, whereas the products of the earlier patents exhibited haze. Beall et al. explained that the essential absence of haze in their products was due to the very small size of the surface crystals developed in their products, resulting from the very rapid nucleation of their glasses because of the generally lower levels of Li.sub.2 O and Al.sub.2 O.sub.3 therein as compared to the Olcott et al. glasses.
As can be seen from the above disclosures, research has been quite extensive to develop high strength, transparent, surface-crystallized glass articles, wherein the crystals exhibit a lower coefficient of thermal expansion than that of the glass, thereby producing an integral surface layer of compressive stresses on the article. Surprisingly, however, commercial marketing of such articles has been minimal. Three factors have been defined to explain this lack of commercial success:
First, the development of excessive internal tension in the glass is hazarded which can result in forceful fragmentation upon fracture of the articles, such forceful fragmentation being termed frangibility; PA1 Second, delayed and, hence, unpredictable breakage of the articles has been experienced; and PA1 Third, whereas the articles demonstrate high unabraded mechanical strengths, the layers of crystals have commonly been quite thin, viz., about 10 microns in thickness, such that this strength is dramatically reduced when the surface is subjected to even minimal physical abuse. PA1 (1) The differences in refractive index between the glass and the crystals should be minimized. PA1 (2) The birefringence of the crystals should be low. The birefringence of .beta.-quartz solid solution crystals is intrinsically low, reaching a minimum at about 70% by weight SiO.sub.2. It will be appreciated, of course, that transparent articles can result where the crystals contained therein have dimensions so small as to approach the wavelengths of visible light. However, such articles have not been produced in surface-nucleated, thick-skinned bodies.
Accordingly, the primary objective of the present invention was to devise compositions for glass articles upon which, when heat treated, an integral crystalline surface compression layer can be generated, said articles exhibiting high strength, virtually haze-free transparency, and relatively gentle fragmentation upon fracture, and said surface layer being of sufficient thickness to essentially eliminate delayed breakage of the articles.